The present invention relates to a process for the preparation of a bisphenol A which is light in colour and has low residual contents of oxygen and phenol, starting from mixed crystals of bisphenol A and phenol.
EP-A 523 931 discloses a process wherein an adduct of bisphenol A and phenol is melted in an atmosphere containing 50 ppm at most, preferably 10 ppm at most, of oxygen. The melting is carried out at 115xc2x0 C. to 180xc2x0 C., preferably 120xc2x0 C. to 150xc2x0 C., at a pressure of 1.0 to 5.0 atm., preferably 1.0 to 1.9 atm. Phenol is removed from the melt by evaporation. The temperature is 160xc2x0 C. to 200xc2x0 C., preferably 170xc2x0 C. to 185xc2x0 C.; the pressure is at most 100 torr (133 mbar), preferably 5 to 40 torr (7 to 53 mbar). Preferably, the bulk of the phenol is first of all distilled off in a downflow evaporator at 160xc2x0 C. to 185xc2x0 at 20 to 80 mbar to a residual content of 1 to 5wt. %, and the residual phenol is then removed by steam at 20 mbar at most and at 170xc2x0 C. to 185xc2x0 C. The ratio of steam:BPA is from 1:50 to 1:5, preferably 1:25 to 1:10. In order to effect the separation of the phenol from the BPA as far as possible with the exclusion of oxygen, it is proposed that all the surfaces of the apparatus used which are in contact with the product be freed from oxygen by means of an organic solvent, preferably phenol. The best bisphenol products obtained from the directions given in EP-A 523 931 have colour values (APHA) of 10.
A process has now been found whereby products having even better colour values can be obtained, and which moreover renders unnecessary the elaborate washing step for removing traces of oxygen from the apparatus.
The invention provides a process for the preparation of bisphenol A, wherein a mixed crystallisate of bisphenol A and phenol is melted at temperatures of 100xc2x0 C. to 120xc2x0 C., the melt is then rendered inert with nitrogen and fed in continuously at the top of a first distillation unit wherein, at temperatures of 120xc2x0 C. to  less than 160xc2x0 C., preferably 120xc2x0 C. to 157xc2x0 C., and at pressures of  greater than 80 to 200 mbar, preferably 85 to 200 mbar, particularly preferably 85 to 120 mbar, the phenol content of the melt is decreased to 10 to 25 wt. %, and at the same time 0.5 to 50 vol. %, preferably 2 to 20 vol. %, of nitrogen, based on the volume of introduced melt, is passed in via the bottom of the distillation unit, and the concentrated melt freed from oxygen is subsequently freed from residual phenol at temperatures of 180xc2x0 C. to 220xc2x0 C. and at 1 to 1.5 bar nitrogen partial pressure.
The invention also provides a process for the preparation of melts containing 75 to 90 wt. % bisphenol A and 10 to 25 wt. % phenol and having an oxygen content of  less than 1 ppm, preferably  less than 100 ppb, particularly preferably  less than 10 ppb, wherein mixed crystals containing 60 wt. % bisphenol A and 40 wt. % phenol are melted at temperatures of 100xc2x0 C. to 120xc2x0 C., the melt is then rendered inert with nitrogen and fed in continuously at the top of a distillation unit wherein, at temperatures of 120xc2x0 C. to  less than 160xc2x0 C., preferably 120xc2x0 C. to 157xc2x0 C., and at pressures of  greater than 80 to 200 mbar, preferably 85 to 200 mbar, particularly preferably 85 to 120 mbar, the phenol content of the melt is decreased to 10 to 25 wt. %, and at the same time 0.5 to 50 vol. % of nitrogen, preferably 2 to 20 vol. %, of nitrogen, based on the volume of introduced melt, is passed in via the bottom of the distillation unit.
In the process according to the invention, mixed crystallisates of bisphenol A and phenol are used as educt, preferably those having a bisphenol content of 60 wt. % and a phenol content of 40 wt. %. Such mixed crystallisates are obtained, for example, during the acid-catalysed reaction of phenol and acetone, which is described, for example, in U.S. Pat. No. 2,775,620 or EP-A 342 758. As a rule, the preparation is conducted as a continuous process. The mixed crystallisate of bisphenol A and phenol obtained as product is separated off by filtering the mother liquor, for example, by means of a rotary filter. The filter cake can then be washed with phenol in order to remove adhering impurities. The resulting cake of mixed crystals can be used as educt in the process according to the invention.
In a first step, the mixed crystallisate is melted at temperatures of 100xc2x0 C. to 120xc2x0 C. This is carried out in a nitrogen atmosphere. Because of the low reactivity of bisphenol with oxygen at these low temperatures, it is not necessary here to ensure that oxygen is totally excluded. Oxygen contents of 0.1 to 2 vol. % can therefore be tolerated in the atmosphere during the melting process. It is advisable to collect the melt in a container rendered inert with nitrogen.
The melt, which may still contain dissolved oxygen, is then fed in continuously at the top of a distillation unit. It is important that as large an exchanging surface as possible be produced in this distillation unit. Accordingly, a packed column, packing column, tray column or a downflow evaporator, film evaporator or an evaporator with forced circulation is preferably used for this step.
In the distillation unit, at temperatures of 120xc2x0 C. to  less than 160xc2x0 C., preferably 120xc2x0 C. to 157xc2x0 C., and at pressures of  greater than 80 to 200 mbar, preferably 85 to 200 mbar, particularly preferably 85 to 120 mbar, the phenol content of the melt is decreased to 10 to 25 wt. %, and at the same time 0.5 to 50 vol. % of nitrogen, preferably 2 to 20 vol. %, of nitrogen, based on the volume of introduced melt, is passed in via the bottom of the distillation unit.
It is important in this step that melt and nitrogen be passed through in countercurrent, in order to discharge the dissolved oxygen in the melt as completely as possible. Here the removal of the oxygen is assisted by the evaporation of phenol and by the lowered pressure. Under the given conditions, the stream of nitrogen assists the removal of oxygen without lowering the partial pressure of the phenol. In the first distillation unit, dissolved oxygen is removed from the melt under mild conditions, leaving a residual content of  less than 1 ppm. Melts having oxygen contents of  less than 100 ppb and even  less than 10 ppb can be obtained by optimal selection of the conditions. Moreover, no losses of bisphenol A occur under the conditions of pressure and temperature established in this step; such losses would necessitate an additional processing step in which bisphenol A is recovered from the vapour phase.
In a further step, the melt thus obtained can then be freed from residual phenol at temperatures of 180xc2x0 C. to 220xc2x0 C., preferably 185xc2x0 C. to 195xc2x0 C., and nitrogen partial pressures of 1 to 1.5 bar, preferably 1 to 1.1 bar. In the course of this, a stream of nitrogen is passed through the melt in order to assist the removal of the phenol (xe2x80x9cstrippingxe2x80x9d). Here the proportion of gas to melt is preferably 10 to 1000 m3 nitrogen per t of melt. The units employed for the removal of phenol are familiar to the person skilled in the art; for example, a packed column operated by flooding can be used for this.
The removal of phenol at atmospheric pressure enables the melt to be rendered inert better than is possible by the desorption under vacuum conditions as described in EP-A 523 931 and has the advantage, moreover, that no oxygen can flow into the apparatus in the event of leakages. Furthermore, the apparatus is rendered inert by the stream of nitrogen and any oxygen which may be adsorbed onto the surfaces in contact with the product is swept out, which renders washing of these surfaces with organic solvent superfluous. The bisphenol A thus obtained has a residual phenol content of 50 ppm at most and shows colour values (measured in accordance with ASTM D 1686) of less than 10.
The melt can subsequently be processed directly into pellets or used for the preparation of sodium bisphenolate solutions for polycarbonate production. Polycarbonates produced from bisphenol A obtained by the process according to the invention have a lower Yellowness Index (Y.I.) than that of the products of prior art. The bisphenol A obtained by the process according to the invention can also be used with advantage for the preparation of epoxy resins.